Facial Eczema in Deer: Sporidesmin Toxicosis, Photosensitization, and Skin Injury

Facial eczema in deer is a pasture-associated toxicosis, not a true eczema or primary skin allergy. It happens when deer eat spores from the fungus Pithomyces chartarum, which can grow on dead plant material in pasture. The fungus produces sporidesmin, a toxin that is absorbed from the gut and concentrated in the liver and biliary system.

The main injury is liver and bile duct damage. When bile flow is impaired, a photodynamic pigment called phylloerythrin is not cleared normally. That pigment then circulates in the body and makes sun-exposed, lightly haired, or lightly pigmented skin much more sensitive to ultraviolet light. In deer, this can lead to painful inflammation and skin injury on the face, ears, eyelids, muzzle, and sometimes the tongue.

Despite the name, the condition can affect more than the face. Farmed deer may show widespread photosensitization, reduced grazing, poor growth, and chronic liver scarring after significant exposure. Young animals can be affected severely, and herd outbreaks may follow warm, humid weather that favors fungal growth on pasture litter.

Because skin lesions are only part of the problem, treatment and prevention focus on the whole animal, especially liver support, shade, pasture management, and reducing further toxin exposure. Your vet can help determine how serious the liver injury may be and which care tier best fits the deer and the farm situation.

The underlying cause is ingestion of sporidesmin, a toxin produced by the saprophytic fungus Pithomyces chartarum. This fungus grows on dead pasture litter, especially during warm, humid periods and after warm rains. Risk tends to rise in late summer and autumn in regions where environmental conditions support fungal growth.

Sporidesmin is absorbed from the gastrointestinal tract and concentrated in the liver and bile ducts, where it causes inflammation, tissue injury, and impaired bile flow. Once bile excretion is disrupted, phylloerythrin from chlorophyll breakdown accumulates in the bloodstream. Sunlight then activates that compound in exposed skin, causing hepatogenous photosensitization.

Pasture conditions matter. Dense litter, certain improved pastures, and weather that favors fungal sporulation can all increase exposure. Deer do not need to eat visibly moldy feed to become affected. The problem is often tied to grazing conditions rather than a single obvious feed error.

Other diseases can also cause crusting, hair loss, or facial lesions in deer, including parasites, bacterial skin disease, trauma, and other causes of liver dysfunction. That is why your vet may recommend testing rather than assuming every sun-sensitive skin lesion is facial eczema.

Diagnosis usually combines history, pasture risk, clinical signs, and laboratory findings. Your vet will look for a pattern of recent grazing exposure during a high-risk weather period, followed by photosensitization signs such as inflamed ears, facial crusting, and shade-seeking behavior. In deer, the exam also focuses on body condition, hydration, pain level, and whether multiple animals are affected.

Bloodwork is often very helpful. Deer with facial eczema commonly have markedly increased liver enzymes, and chemistry changes may support bile duct and liver injury. Depending on the case, your vet may also assess bilirubin, total protein, hydration status, and other indicators of systemic illness.

When the diagnosis is uncertain, your vet may recommend pasture spore counts, skin or liver sampling, or necropsy of a deceased herd mate. Histopathology can show bile duct injury, fibrosis, and other lesions that fit sporidesmin toxicosis. These steps can be especially useful in herd outbreaks, chronic poor performance, or when other skin diseases are also possible.

A practical diagnosis often means answering two questions: Is this photosensitization? and what caused the liver injury? Your vet can help separate facial eczema from primary sunburn, contact irritation, plant-related photosensitization, or infectious skin disease.

Prevention centers on reducing sporidesmin exposure before clinical signs appear. The highest-risk periods are warm, humid stretches that favor fungal growth on dead pasture litter. During those times, work with your vet and farm advisors to identify risky paddocks, monitor herd behavior closely, and move deer early if shade-seeking or skin sensitivity starts to appear.

Pasture management matters. Lowering dead litter burden, avoiding prolonged grazing of high-risk paddocks during danger periods, and using safer forage strategies can all help reduce exposure. In regions where facial eczema is a known problem, some farms use pasture spore monitoring to guide movement and management decisions.

Good shade access is also important. Shade does not prevent the liver injury, but it can reduce the severity of sun-triggered skin damage once photosensitization is developing. Deer with any suspicion of facial eczema should be protected from strong sunlight while your vet evaluates them.

Ask your vet about a herd-level prevention plan if you keep farmed deer in an area with seasonal risk. That plan may include pasture surveillance, nutrition review, early removal from suspect paddocks, and protocols for monitoring young or more vulnerable animals. Prevention is usually far more effective than trying to manage advanced liver injury after the fact.